Evaluation of soil moisture content and electrical conductivity are two of the parameters necessary for efficient control of irrigation, fertilization of crops and turf production and maintenance. Presently, the most effective and widely used means of assessing soil moisture content is through the use of Time Domain Reflectometry (TDR), which assesses the dielectric constant of soil responsive to an electrical signal travelling into the soil along a conductor, the signal reflected back towards the transmitter. Over the past 20 years many researchers have proven the accuracy of assessing soil moisture content through the estimation of dielectric constant.
Using traditional TDR measurements has a disadvantage in that creating and receiving the TDR signal requires expensive circuitry to construct the output and receive signals. Several inventions such as U.S. Pat. No. 5,818,214 issued Oct. 6, 1998 to Pelly et al, the entire contents of which is incorporated herein by reference, have been developed which have attempted to reduce the cost and complexity of transmitting a TDR signal with marginal improvements.
TDR technology utilizes an extremely fast rise time pulse which is transmitted through an open ended wave guide structure. The pulse promulgates down the wave guide structure and the soil it is in contact with and the corresponding return signal is delayed by the dielectric constant of the soil. The generation of the TDR signal and the equipment required to analyze the return signal has limited the use of this technique to research and scientific applications.
Other patents have attempted to reduce cost and have offered alternative methods to measure the dielectric constant of soils. U.S. Pat. No. 5,148,125 issued Sep. 15, 1992 to Woodhead et al, the entire contents of which is incorporated herein by reference, utilizes a buried transmission line coupled to an oscillator. The buried transmission line forms part of a feedback loop of the amplifier, and the resultant frequency of the circuit is responsive to the dielectric constant of the material in which the transmission line is embedded. This method is reliable under laboratory conditions where environmental conditions and homogeneity of the material being tested can be tightly controlled, but is not overly successful in the field.
Other sensors have employed methods which measure the resistance and capacitance of the soils through the use of specialized probes coupled to either timing or resonance circuits such as U.S. Pat. No. 5,341,673 issued Aug. 30, 1994 to Burns et al, the entire contents of which is incorporated herein by reference. The main limitation with these sensor approaches is the susceptibility of soil salinity to influence the measurements, thus degrading the repeatability of accurate soil moisture content when fertilization contents are applied to the subject fields or plots. The various mentioned sensors operate at frequency ranges below 300 MHz, and thus do not take advantage of the tendency of microwave frequencies to reduce the dependence of measurement on soil salinity.
U.S. Pat. No. 2,659,860 issued Nov. 17, 1953 to Breazeale, the entire contents of which is incorporated herein by reference, describes a method of measurement of moisture content of materials by propagating a 10 GHz microwave signal in a thru path configuration and determining the moisture content through the measurement of the attenuation through the material.
U.S. Pat. No. 4,361,801 issued Nov. 30, 1982 to Meyer et al, the entire contents of which is incorporated herein by reference, describes a technique that uses a 9 GHz signal to measure both the attenuation and phase delay to calculate the moisture content independent of material density. Meyer et al, determine the Volumetric Water Content by measuring the magnitude and phase information and generating a complex quantity which represents the dielectric constant and complex permittivity of the material under test. This method is disadvantageous due to the costs associated with the signal reception and generation components.
U.S. Pat. No. 6,147,503 issued Nov. 14, 2000 to Nelson et al, the entire contents of which is incorporated herein by reference, describes a method that is also independent of a narrow range of densities using a propagation frequency of 11.3 GHz and 18 GHz to calculate the permittivity for the determination of the moisture content of the materials under test. U.S. Pat. Nos. 6,476,619 issued Nov. 5, 2002 to Moshe et al; 6,111,415 issued Aug. 29, 2000 to Moshe; 5,845,529 issued Dec. 8, 1998 to Moshe et al; 6,107,809 issued Aug. 22, 2000 to Moshe et al; and 4,361,801 issued Nov. 30, 1982 to Meyer et al, the entire contents of each of which are incorporated herein by reference, all typically operate at microwave frequencies above 7 GHz. Again disadvantageously, the costs associated with the generation and reception of the magnitude and phase components of the signals are very high.
U.S. Pat. No. 7,135,871 issued Nov. 14, 2006 to Pelletier, the entire contents of which is incorporated herein by reference, describes a method which generates a varying microwave frequency with the suggested use of an oven stabilized VCO to produce the primary microwave frequencies of 1.8 GHz to 2.5 GHz to determine the dielectric constant and complex permittivity and suggests the use of multiple VCOs or an Ultra Wide Band VCO spanning multiple octaves. The use of oven stabilized VCOs and the requirement of a VCO of multiple octaves in band width, to determine the electrical conductivity and the dielectric constant and complex permittivity are expensive to procure and difficult to develop without significant monetary costs. An alternative solution for a low cost, accurate technique for the determination of moisture content and electrical conductivity is thus desired.